New simulations published in The Astrophysical Journal reveal that gas giant planets like Jupiter and Saturn can build solid 10-Earth-mass cores in roughly 200,000 years. According to researchers Hiroshi Kobayashi of Nagoya University and Hidekazu Tanaka of Tohoku University, icy pebbles drifting inward from the outer protoplanetary disk collide and convert into planetesimals. This creates a dense reservoir between 6 and 9 astronomical units (au) that growing planetary embryos can sweep up rapidly before gas disks disappear.
Building a giant planet poses a timing challenge for planetary scientists. A world like Jupiter needs to accumulate a massive solid core while plenty of hydrogen and helium still surround its young host star. Traditional models face obstacles. Growing a critical core of about 10 Earth masses by collecting 10-kilometer planetesimals can take roughly 10 million years. Yet, protoplanetary gas disks typically survive for only several million years.
The Efficiency Problem of Direct Pebble Accretion
Pebble accretion emerged as a solution because planetary embryos capture fast-moving pebbles much more efficiently than widely scattered large rocks. However, Kobayashi and Tanaka identified an efficiency roadblock in standard models. According to their findings, a single planetary core captures less than 10% of the pebbles passing its orbit. Building a 10-Earth-mass core through direct pebble accretion alone would require several hundred Earth masses of solid material flowing through the disk, a density most protoplanetary disks appear not to possess.
Did you know? One astronomical unit (au) equals the average distance between the Earth and the Sun. Jupiter currently orbits our Sun at about 5.2 au, while Saturn sits near 9.5 au.
Icy Pebbles Create a Dense Planetesimal Reservoir
To solve the timing and efficiency paradox, the researchers built a dust-to-planet simulation spanning 3 to 108 au. As microscopic dust coagulated into porous aggregates, gas drag caused pebble-sized material from the outer disk to drift inward. Once these icy pebbles entered the inner disk, collisions converted them into larger planetesimal precursors and objects measuring 100 meters to 10 kilometers across.
Because these larger planetesimals feel far less gas drag than pebbles, their inward migration slowed down drastically. Solid material piled up between approximately 6 and 9 au, increasing the solid surface density by about a factor of 10. At roughly 7 au, the simulated surface density climbed to about 20 grams per square centimeter, far exceeding the roughly 3 grams per square centimeter needed to form a core.
Outpacing Planetary Migration Timelines
Growing planets face another clock besides the evaporation of the gas disk. Gravitational interactions with surrounding material trigger Type I migration, which can pull massive embryos toward their star on timescales that can approach 100,000 years. Kobayashi and Tanaka’s simulation showed that planetary embryos exceeding 10 Earth masses appeared near 6 to 7 au after approximately 200,000 years.
Once the concentration of smaller planetesimals developed, the immediate core-growth timescale fell to roughly 20,000 years. This rapid assembly outpaces the Type I migration timescale, allowing the core to secure its massive gas envelope before drifting too close to the star.
“We already know quite a bit about how planets are made,” Kobayashi told The Brighter Side of News. “Despite everything we know, the formation of gas giants, like Jupiter and Saturn, has long baffled scientists.”
Implications for Solar System Formation and Exoplanets
The simulation results place the final formation zone of these giant planet cores close to Jupiter’s actual home. After rapid gas accretion and subsequent orbital movement, a Jupiter-mass planet could potentially move somewhat inward from that starting location to near its current 5.2-au orbit. The researchers also noted that disk composition dictates the outcome. Lower solid-to-gas ratios shift giant-planet cores closer in, around 3 to 4 au, while stretching the timeline to approximately 300,000 years.
Disks containing less than about 100 Earth masses of solids struggled to form giant planets quickly enough. Understanding these formation mechanics helps astronomers decode how habitable worlds emerge and survive in stable orbits across the galaxy.
“We expect our research will help lead to the full elucidation of the origin of habitable planets, not only in the solar system, but also in other planetary systems around stars,” Kobayashi said.
Frequently Asked Questions
How fast do gas giant cores form in the new model?
Simulations show that solid cores of about 10 Earth masses can form in approximately 200,000 years when drifting icy pebbles convert into concentrated planetesimals.
Why did traditional pebble accretion fall short?
Single planetary cores capture less than 10% of the pebbles passing their orbits. Direct pebble accretion requires several hundred Earth masses of solid material flowing through the disk, which is rarely available.
Where do gas giant cores grow in the simulation?
The most efficient core growth occurs between 6 and 9 astronomical units, closely mirroring the birthplace of Jupiter in our solar system.